William was the son of the unmarried Robert I, Duke of Normandy, by Robert's mistress Herleva. His illegitimate status and his youth caused some difficulties for him after he succeeded his father, as did the anarchy that plagued the first years of his rule. During his childhood and adolescence, members of the Norman aristocracy battled each other, both for control of the child duke and for their own ends. In 1047 William was able to quash a rebellion and begin to establish his authority over the duchy, a process that was not complete until about 1060. His marriage in the 1050s to Matilda of Flanders provided him with a powerful ally in the neighbouring county of Flanders. By the time of his marriage, William was able to arrange the appointments of his supporters as bishops and abbots in the Norman church. His consolidation of power allowed him to expand his horizons, and by 1062 William was able to secure control of the neighbouring county of Maine.
In the 1050s and early 1060s William became a contender for the throne of England, then held by the childless Edward the Confessor, his first cousin once removed. There were other potential claimants, including the powerful English earl Harold Godwinson, who was named the next king by Edward on the latter's deathbed in January 1066. William argued that Edward had previously promised the throne to him and that Harold had sworn to support William's claim. William built a large fleet and invaded England in September 1066, decisively defeating and killing Harold at the Battle of Hastings on 14 October 1066. After further military efforts William was crowned king on Christmas Day 1066, in London. He made arrangements for the governance of England in early 1067 before returning to Normandy. Several unsuccessful rebellions followed, but by 1075 William's hold on England was mostly secure, allowing him to spend the majority of the rest of his reign on the continent.
William's final years were marked by difficulties in his continental domains, troubles with his eldest son, and threatened invasions of England by the Danes. In 1086 William ordered the compilation of the Domesday Book, a survey listing all the landholdings in England along with their pre-Conquest and current holders. William died in September 1087 while leading a campaign in northern France, and was buried in Caen. His reign in England was marked by the construction of castles, the settling of a new Norman nobility on the land, and change in the composition of the English clergy. He did not try to integrate his various domains into one empire but instead continued to administer each part separately. William's lands were divided after his death: Normandy went to his eldest son, Robert Curthose, and his second surviving son, William Rufus, received England.
Norsemen first began raiding in what became Normandy in the late 8th century. Permanent Scandinavian settlement occurred before 911, when Rollo, one of the Viking leaders, and King Charles the Simple of France reached an agreement surrendering the county of Rouen to Rollo. The lands around Rouen became the core of the later duchy of Normandy. Normandy may have been used as a base when Scandinavian attacks on England were renewed at the end of the 10th century, which would have worsened relations between England and Normandy. In an effort to improve matters, King Æthelred the Unready took Emma of Normandy, sister of Duke Richard II, as his second wife in 1002.
Danish raids on England continued, and Æthelred sought help from Richard, taking refuge in Normandy in 1013 when King Swein I of Denmark drove Æthelred and his family from England. Swein's death in 1014 allowed Æthelred to return home, but Swein's son Cnut contested Æthelred's return. Æthelred died unexpectedly in 1016, and Cnut became king of England. Æthelred and Emma's two sons, Edward and Alfred, went into exile in Normandy while their mother, Emma, became Cnut's second wife.